1. Technical Field
The present invention relates to a tunable optical filter, and more particularly to a dynamic optical filter, such as a reconfigurable blocking filter having a multi-dimensional array of micromirrors to selectively delete individual channels within a wavelength division multiplexed (WDM) optical signal.
2. Description of Related Art
MEMS micro-mirrors have been widely explored and used for optical switching and attenuation applications. The most commonly used application is for optical cross-connect switching. In most cases, individual micro-mirror elements are used to ‘steer’ a beam (i.e., an optical channel) to a switched port or to deflect the beam to provide attenuation on a channel-by-channel basis. Each system is designed for a particular ‘wavelength plan’ —e.g. “X” number of channels at a spacing “Y”, and therefore each system is not ‘scalable’ to other wavelength plans.
Further, dynamic gain equalization (or “flattening”) is a critical technology for deployment of next-generation optical network systems. Dynamic gain equalizing filters (DGEF's ) function by adding varying amounts of attenuation at different spectral locations in the signal spectrum of optical fiber communication systems. For instance, a DGEF may be designed to operate in the “C-band” (˜1530-1565 nm) of the communication spectrum that is capable of selectively attenuating spectrally concatenated “bands” of some preselected spectral width (e.g., 3 nm). The total number of bands within the DGEF is determined by the width of an individual band.
In the networking systems, it is often necessary to route different channels (i.e., wavelengths) between one fiber and another using a reconfigurable optical add/drop multiplexer (ROADM) and/or an optical cross-connect device. Many technologies can be used to accomplish this purpose, such as Bragg gratings or other wavelength selective filters.
One disadvantage of Bragg grating technology is that it requires many discrete gratings and/or switches, which makes a 40 or 80 channel device quite expensive. A better alternative would be to use techniques well known in spectroscopy to spatially separate different wavelengths or channels using bulk diffraction grating technology. For example, each channel of an ROADM is provided to a different location on a generic micro-electro-mechanical system (MEMS) device. The MEMs device is composed of a series of tilting mirrors, where each discrete channel hits near the center of a respective mirror and does not hit the edges. In other words, one optical channel reflects off a single respective mirror.
One issue with the above optical MEMs device is that it is not “channel plan independent”. In other words, each MEMs device is limited to the channel spacing (or channel plan) originally provide. Another concern is that if the absolute value of a channel wavelength changes, a respective optical signal may begin to hit an edge of a corresponding mirror leading to large diffraction losses. Further, since each channel is aligned to an individual mirror, the device must be carefully adjusted during manufacturing and kept in alignment when operated through its full temperature range in the field.
It would be advantageous to provide an optical blocking filter that mitigates the above problems by using an array of micro-mirrors.